Interference-proof reception of radio signals using frequency hopping techniques

Abstract

Two interference-proof techniques are shown for detecting binary radio sils transmitted by frequency hopping techniques. In the first, the decision as to which type of binary signal is being received is made by first comparing the energy on each mark and space channel with the energy contents of the same channels during a preceding baud which is blank except for jamming and interference, thus obtaining mark and space energy differences. The larger of these differences determines the type of signal received. In the other technique, the energy contents of both mark and space channels are compared with the energies received simultaneously on a nearby blank channel.

What is claimed is: 
     
       1. A receiver for binary radio signals which are transmitted in the frequency hopping mode in which the mark and space frequencies are changed from baud to baud in accordance with a pseudo-random code, comprising, means to compare the energy contents of each mark and space channel with the energy contents of the same mark and space channels during the preceding blank baud, said means comprising comparator circuits for obtaining the absolute value of the difference between the energies of the current mark and space channels and stored values of the energy of the same channels during the preceding blank baud, and further comparison means connected to the outputs of said comparator circuits for producing a binary signal corresponding to the larger of said absolute values. 
     
     
       2. A receiver for binary radio signals which are transmitted in the frequency hopping mode in which the mark and space frequencies are changed from baud to baud in accordance with a pseudo-random code, comprising, common radio frequency circuitry, four sections having their inputs connected in parallel to the output of said radio frequency circuitry, two of said sections being tuned during any given baud to the current mark and space channel assignments of that baud and the other two of said sections being tuned to the channel assignments of the next succeeding baud, the tuning of each pair of said sections remaining constant during two successive bauds, each said section comprising comparison means for comparing the energy received during said given baud at the current channel assignment thereof with the energy received on the same channel during the preceding baud, resulting in mark and space energy differences, and further comparison means for comparing said mark and space energy differences, the larger of the two determining the type of signal received during said given baud. 
     
     
       3. The receiver of claim 2 wherein each said section comprises, a mixer, a bandpass filter and an integrate and dump circuit connected in cascade and a pair of energy stores connected to the output of each said integrate and dump circuits, said comparison means having a pair of inputs connected to the outputs of each pair of energy stores, and said further comparison means comprising a pair of comparator circuits each having two inputs and a single output, said last-named inputs being connected to the comparison means output of two of said sections. 
     
     
       4. The receiver of claim 3 wherein each of said mixers has an oscillator connected thereto, and a key generator connected to all of said oscillators, said key generator arranged to control the frequencies of said oscillators in accordance with a code stored in said key generator. 
     
     
       5. A method of detecting binary radio signals which are transmitted in the frequency hopping mode, comprising the steps of, first comparing the energy contents of each channel during the blank baud prior to the actual transmission with the energy content of each channel during the baud when transmission is expected, said first step resulting in the absolute value of the difference of said energies for both the mark and space channels and further comparing said differences of said energies of said mark and space channels and producing a binary signal corresponding to the larger of said two differences. 
     
     
       6. A receiver for binary radio signals which are transmitted in the frequency hopping mode comprising, means to compare and calculate the absolute value of the energy difference between the mark channel for the current baud and the energy received simultaneously on a nearby blank channel, similar means to compare and calculate the absolute value of the energy difference between the space channel for said current baud and the energy received simultaneously on another nearby blank channel, thereby obtaining mark and space energy differences, and means to produce a binary signal corresponding, to the larger of said mark and space energy differences. 
     
     
       7. A receiver for binary radio signals which are transmitted in the frequency hopping mode comprising, common broadband radio frequency circuitry, mark and space mixers connected in parallel to the output of said radio frequency circuitry, a first pair of bandpass filters connected in parallel to the output of said mark mixer, a second pair of bandpass filters connected in parallel to the output of said space mixer and integrate and dump circuits connected to the outputs of each of said filters, first comparator means connected to the outputs of the two integrate and dump circuits which are in cascade with said mark mixer and a second similar comparator means connected to the outputs of the other two of said integrate and dump circuits, and further comparator means connected to the outputs of said first and second comparator means. 
     
     
       8. The receiver of claim 7 wherein a mark oscillator is connected to said mark mixer and a space oscillator to said space mixer, and key generator connected to said oscillators and arranged to tune said oscillators such that one of said first pair of filters will receive the mark channel of the current baud and one of said second pair of filters will receive the space channel of the current baud, the other two filters being tuned to a nearby blank channel. 
     
     
       9. The method of detecting binary radio signals which are transmitted in the frequency hopping mode, comprising the steps of comparing and calculating the absolute value of the energy difference between the mark channel for the current baud and a nearby blank channel and simultaneously comparing and calculating the absolute value of the energy difference between the space channel for the current baud and a nearby blank channel, thereby obtaining mark and space energy differences, and producing a binary signal corresponding to the larger of said mark and space energy differences.